Recordable DVDs such as DVD+RW, DVD+R, DVD-R, DVD-RW and DVD-RAM are commercially available in addition to playback-only optical recording media such as DVD-ROM. The DVR+R and DVD+RW are developed on the basis of conventional CD-R and CD-RW (recording compact disc) such that the recording density (track pitch, signal mark length) and substrate thickness are compatible between CD and DVD conditions in order to assure the reproduction compatibility with playback-only DVD.
For example, DVD+Rs have a construction, similarly with CD-Rs, that an optical recording layer is provided on a substrate by way of spin-coating a cyanine or an azo dye, an information recording substrate with a light reflective layer of a metal film is laminated through an adhesive at the back side of the optical recording layer.
In the construction, dye substances or the compounds are typically utilized to form the optical recording layer or the dye recording layer. CD-Rs have specific reflectance as high as 65% or more as defined by specifications in terms of CDs. In order that such a construction exhibits higher reflectance, the optical absorption layer should represent a specific range of complex refractive index at the recording-reproducing wavelength, and the dye should represent a proper optical absorption property, as also been required for DVDs.
On the other hand, there is proposed a read-only DVD with two layers of image recording layer as the recording unit in order to increase the recording capacity. A translucent layer of a first information recording layer is formed at inner surface of a first substrate, and a reflective layer of a second information recording layer is formed at inner surface of a second substrate. The translucent layer is formed of a dielectric or thin-metal film, and the reflective layer is formed of a metal film.
Convexo-concave recording marks are formed at the substrate surface on which the translucent layer and the reflective layer are formed; the first substrate and the second substrate are laminated through a transparent intermediate layer of a UV-ray curable resin, thereby recording signals can be read by making use of reflection-interference effects on reproducing laser light incident from the transparent side of the first substrate.
Since signals can be read from two information recording layers, the recording capacity may be up to about 8.5 GB. The thicknesses of the first and second substrates are about 0.6 mm respectively, and the thickness of the transparent intermediate layer is about 50 μm.
The first information recording layer is designed to have a reflectance of about 30%; the laser light for reproducing the second information recording layer is reflected at the second information recording layer after decaying about 30% of the initial light amount at the first information recording layer, then further decaying at the first information recording layer, and emitting from the disc. A laser light of reproducing light is narrowed to focus at the first or second information recording layer and the reflected light is detected, thereby the signals at the information recording layer can be reproduced. The wavelength of the laser light for information reproduction is about 650 nm in cases of DVDs.
Recently, a recordable DVD has been developed that has two dye-recording layers, in which a first dye recording layer and a first translucent reflective layer are laminated on a first substrate, on which grooves having been formed, to form a first information recording layer; a UV ray curable resin is coated on the first information recording layer, and a stamper is contacted to the uncured resin; the resin is cured by UV irradiation and peeled thereby to transfer grooves of a second recording layer is transferred to which a second dye recording layer and an light reflective layer is laminated to form a second information recording layer, then the entire is laminated to a second substrate. The two-layer recordable DVD is commercially available as 2P (Photo Polymerization) type.
In addition, a two-layer recordable DVD of IS (Inversed Stack) type is also commercially available, which is produced in a manner that a first dye recording layer and a first translucent reflective layer are laminated on a first substrate, on which grooves having been formed, to form a first information recording layer; a light reflective layer, a second dye recording layer, and a protective layer are laminated on a second substrate, on which grooves having been formed, to form a second information recording layer; then these laminates are further laminated by use of a UV curable resin.
Patent Literature 1 discloses an optical recording medium having a construction that a second barrier layer is provided between an intermediate or adhesive layer and a second optical absorption layer; in Examples thereof, only Au is exemplified as the material of the second barrier layer.
However, employment of metals tends to increase the absorption coefficient “k” of complex refractive index (n−ik), which typically makes impossible to take high reflectance and sufficient modulation degrees when being recorded on two image recording layers. Moreover, the literature describes in paragraph 0039 that the material of the second barrier layer preferably has “k” of no less than 0.1; as such, the literature describes no technical idea in relation to absorption coefficient as low as below 0.05 as described in the present invention.
Patent Literature 2 discloses an optical recording medium that has a laminate structure between a first recording disc and a second recording disc, in which the first recording disc is formed by laminating an organic dye recording layer and an light reflective layer on an optical transparent substrate and the second recording disc is formed by laminating an optical reflective layer, an organic dye recording layer, and a dye protective layer on an optical transparent substrate; however, no description appears with respect to the dye utilized in the organic dye recording layer.
Patent Literature 3 describes in relation to a thermal deposition temperature and a maximum absorption wavelength of dyes, however, also describes that sufficient modulation degree of recording signals is hardly obtainable when the thickness of the protective layers is below 80 nm.    Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 2000-311384    Patent Literature 2: JP-A No. 2003-303447    Patent Literature 3: JP-A No. 2006-48992